Optical fiber lasers and amplifiers used in the amplification of light usually include optical fibers having optically active waveguiding cores doped with rare-earth active ions, such as for example, thulium, erbium, ytterbium and neodymium. Amplification is achieved by stimulated emission of photons from dopant ions in the doped optical fiber, and so the composition of the optical fiber affects the properties of the emitted amplified light. As such, the composition of the fibers is tailored to optimize the desired properties for a given application. However, it is also important that the optimized desired optical properties of the fiber remain stable over time.
In general, an optical fiber may be multi-mode or single-mode or few-moded like large mode area (LMA) fibers. A multi-mode or LMA fiber allows for more than one mode of the light wave, each mode travelling at a different phase velocity, to be confined to the core and guided along the fiber. A single-mode fiber supports only one transverse spatial mode at a frequency of interest. Given a sufficiently small core or a sufficiently small numerical aperture, it is possible to confine a single mode, the fundamental mode, to the core. Fundamental modes are preferred for many applications because higher beam quality and focusing properties are obtained, and the intensity distribution of the light wave emerging from the fiber is unchanged regardless of launch conditions and any disturbances of the fiber.
A real optical fiber attenuates the light travelling through it, it is therefore advantageous to keep the fiber length as short as possible. It is important to maximize the energy stored in the gain medium. The saturation energy of a gain medium is the energy required to achieve significant saturation of the gain medium. It is also important to maximize the energy extracted from the stored energy. The main limitations for achieving high peak powers inside optical fibers are due to small mode size and large propagation length, which can create non-linear effects. Another problem with some gain media having high-emission cross sections is that the saturation energy per unit area is low, which consequently limits the amount of energy that may be extracted and causes pulse deformation.
Longitudinal mode beating can be an important source of high frequency noise which consequently gives rise to peak power fluctuations in the pulse structure of a pulsed amplifier or laser. Depending on its amplitude and frequency spectrum, this noise can severely limit the ability to generate stable optical pulses having special shapes with fine structures.
Another problem encountered with high-energy amplifiers and lasers are the non-linear effects that appear at high energies. The onset of non-linear effects can severely degrade the spectral content and limit the power output of the laser source.
For some rare-earth doped core compositions, photodarkening (i.e. the formation of photoinduced structural transformations) over time induces losses in the doped glass core of the fibers resulting in reduced output power efficiency of the fibers.
Photodarkening has already been observed in rare-earth-doped silica fibers: M. M. Broer et al. (Opt. Lett. 1993, 18 (10), p. 799-801) describe photodarkening in thulium-doped fibers, M. M. Broer et al. (Opt. Lett. 1991, 16 (18), p. 1391-1393) describe photodarkening in cerium-doped fibers, and E. G. Behrens et al. (1990, JOSA B 7 (8), p. 1437-1444) describe photodarkening in europium- and praseodymium-doped fibers. The first experimental evidence of this phenomenon in ytterbium doped silica fiber was described by R. Paschotta et al. in Opt. Commun., 1997, 136 (5-6), p. 375-378.
U.S. Pat. No. 5,173,456 describes phosphate glass useful in high-energy lasers. Specifically, a low- or no-silica, low- or no-alkali phosphate glass useful as a laser amplifier in a multiple pass, high energy laser system having a high thermal conductivity, low coefficient of thermal expansion, low emission cross section, and a high fluorescence lifetime is described.
There is therefore a need for high-energy lasers and amplifiers with enhanced power output, higher extraction and saturation energy, reduced non-linear effects, reduced need for pulse-shaping, higher beam quality and focusing properties, which are easily pumped and packaged, and which are stable over time.